Polyethylene resin for secondary battery separator, method for producing same, secondary battery separator and secondary battery including same

Abstract

To provide a polyethylene resin for a secondary battery separator that improves processability in an extruder and maintains stiffness of the final separator film by optimizing a content of a low molecular weight polyethylene and a high molecular weight polyethylene.SOLUTION: There is provided a polyethylene resin for a secondary battery separator which has a weight average molecular weight (Mw) value of 350,000 g / mol to 500,000 g / mol through a gel permeation chromatography analysis. The integral area of the low molecular weight region is between 1.3% and 2.4% of the total integral area of the gel permeation chromatography graph, and the integral area of the high molecular weight region is between 9.5% and 13.5%. The low molecular weight region is a segment of the graph where the log M (where M corresponds to the molecular weight of the polymer passing through the column in the gel permeation chromatography) value is 4 or less in the gel permeation chromatography graph, and the high molecular weight region is a segment of the graph where the log M value is 6 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a polyethylene resin for a secondary battery separator, a method for producing the same, and a secondary battery separator and a secondary battery containing the same. [Background technology]

[0002] Ultra high molecular weight polyethylene (UHMWPE) is generally a polymer with a molecular weight of 10 6 g / mol or more, which is further divided into 3×10 6 Ultra-high molecular weight polyethylene resins with a molecular weight of 1000 g / mol or more are called UHMWPE, and those with a molecular weight of less than that are called VHMWPE (very high molecular weight polyethylene). Ultra-high molecular weight polyethylene resins have a higher molecular weight than general-purpose polyethylene, and as such have excellent rigidity, abrasion resistance, environmental stress resistance, uniformity, self-lubrication, chemical resistance, and electrical properties.

[0003] Among these, ultra-high molecular weight polyethylene is widely used as a battery separator due to its excellent chemical resistance and electrical properties. Products with a molecular weight of 250,000 g / mol to 2.5 million g / mol are typically used for battery separators.

[0004] However, as described in US Patent Publication US4972035A, ultra-high molecular weight polyethylene resin is difficult to process due to its low flowability even when fully melted due to its high molecular weight characteristics. Therefore, unlike general-purpose polyethylene, it cannot be pelletized and is sold in the form of powder generated after the polymerization process.

[0005] The produced powder is mixed with oils and uniformly melted and kneaded in a twin screw extruder to form a single phase, and then stretched in the MD (machine direction) and TD (transverse direction) to form a porous film. The phase-separated oil is then removed to complete the battery separator. In this battery separator manufacturing process, stable operation of the melt extrusion process is an important factor in manufacturing separators with uniform physical properties.

[0006] Additionally, with the recent increase in demand for improved stability in secondary battery separators, the importance of maintaining the separator's physical properties and appearance through stable operation during the extrusion process is growing. Poor physical properties mean the separator cannot function properly, while poor appearance can potentially lead to short circuits in the battery. Therefore, along with the development of resin products with optimized processability and rigidity, many extruder conditions (uniform feeding speed, extruder temperature, extruder screw, oil injection temperature, extruder load, etc.) must be optimized. Summary of the Invention [Problem to be solved by the invention]

[0007] One embodiment provides a polyethylene resin for secondary battery separators that improves processability in an extruder and maintains the rigidity of the final separator film by optimizing the content of low molecular weight polyethylene and high molecular weight polyethylene.

[0008] Another embodiment provides a method for manufacturing the polyethylene resin for a separator of a secondary battery. In yet another embodiment, there is provided a separator for a secondary battery, comprising the polyethylene resin for a secondary battery separator.

[0009] In yet another embodiment, a secondary battery including the separator for a secondary battery is provided. [Means for solving the problem]

[0010] In one embodiment, a polyethylene resin for a secondary battery separator is provided, the resin having a weight-average molecular weight (Mw) of 350,000 g / mol to 500,000 g / mol as determined by gel permeation chromatography analysis, an integrated area of ​​a low molecular weight region of 1.3% to 2.4% of the total integrated area of ​​a gel permeation chromatography graph, and an integrated area of ​​a high molecular weight region of 9.5% to 13.5% of the total integrated area of ​​a gel permeation chromatography graph, the low molecular weight region being the portion of the gel permeation chromatography graph where the log M (where M corresponds to the molecular weight of a polymer passing through a column in the gel permeation chromatography) value is 4 or less, and the high molecular weight region being the portion of the gel permeation chromatography graph where the log M value is 6 or more.

[0011] The polyethylene resin may be in powder form. The polyethylene resin may be a particle formed of a low molecular weight polymer having a viscosity average molecular weight of 180,000 g / mol to 700,000 g / mol and a high molecular weight polymer having a viscosity average molecular weight of 1,500,000 g / mol to 2,500,000 g / mol.

[0012] The polyethylene resin may have a viscosity average molecular weight of 200,000 g / mol to 2,500,000 g / mol. The polyethylene resin may have a melting temperature of 130°C to 138°C.

[0013] The polyethylene resin may have an average particle size of 110 μm to 140 μm. The polyethylene resin may have a particle size distribution (SPAN) of 1.0 or less. The molecular weight distribution (MWD) of the polyethylene resin may be 4-6.

[0014] Another embodiment provides a method for producing a polyethylene resin for a separator for a secondary battery, the method including the steps of: injecting ethylene, a catalyst, and an organic solvent into a first reactor; injecting hydrogen into the first reactor; injecting a slurry produced by polymerization in the first reactor into a second reactor; additionally injecting ethylene and an organic solvent into the second reactor; and injecting hydrogen into the second reactor so that a ratio of an injection rate of hydrogen to an injection rate of ethylene into the second reactor is 0.01 to 0.30.

[0015] The method for preparing a polyethylene resin for a secondary battery separator may further include injecting hydrogen into the first reactor so that a ratio of an injection rate of hydrogen to an injection rate of ethylene into the first reactor is 0.02 to 0.35.

[0016] The organic solvent can be an alkane including pentane, hexane, heptane, n-octane, isooctane, or a combination thereof; a cycloalkane including cyclohexane, methylcyclohexane, or a combination thereof; an alkyl aromatic including toluene, xylene, ethylbenzene, isopropylbenzene, ethyltoluene, n-propylbenzene, diethylbenzene, or a combination thereof; a halogenated aromatic including chlorobenzene, chloronaphthalene, orthodichlorobenzene, or a combination thereof; or a combination thereof.

[0017] Yet another embodiment provides a separator for a secondary battery, comprising the polyethylene resin. Yet another embodiment provides a secondary battery including: a positive electrode; a negative electrode; and the separator for a secondary battery located between the positive electrode and the negative electrode. [Effects of the Invention]

[0018] When using a polyethylene resin for a separator of a secondary battery according to an embodiment, the content of low molecular weight polyethylene and high molecular weight polyethylene is optimized to improve processability in an extruder, thereby producing a separator product with a uniform surface, and also to maintain the rigidity of the final separator. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a gel permeation chromatography graph of a polyethylene resin for a secondary battery separator according to an embodiment, illustrating a low molecular weight region and a high molecular weight region of the polyethylene resin for a secondary battery separator according to Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0020] Although the present invention will be described in detail below so that those skilled in the art can easily implement the present invention, it should be understood that the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein.

[0021] One embodiment provides a polyethylene resin for a separator of a secondary battery, which has a weight-average molecular weight (Mw) value of 350,000 g / mol to 500,000 g / mol as determined by gel permeation chromatography, and the integrated area of ​​a low molecular weight region is 1.3% to 2.4% of the total integrated area of ​​a gel permeation chromatography graph, and the integrated area of ​​a high molecular weight region is 9.5% to 13.5% of the total integrated area of ​​a gel permeation chromatography graph, the low molecular weight region being the portion of the gel permeation chromatography graph where the log M value is 4 or less, and the high molecular weight region being the portion of the gel permeation chromatography graph where the log M value is 6 or more, where M corresponds to the molecular weight of a polymer passing through a column in the gel permeation chromatography.

[0022] 1 is a gel permeation chromatography graph illustrating the low molecular weight region and the high molecular weight region of a polyethylene resin for a secondary battery separator according to Example 1, which is an embodiment of the polyethylene resin for a secondary battery separator. Referring to FIG. 1, the portion of the graph on the horizontal axis where the logM value is 4 or less corresponds to the low molecular weight region, and the portion where the logM value is 6 or more corresponds to the high molecular weight region.

[0023] In one embodiment, the polyethylene resin for secondary battery separators may have an integrated area of ​​a low molecular weight region of 1.3% to 2.4%, for example, 1.5% to 2.3%, for example, 1.7% to 2.3%, of the total integrated area in a gel permeation chromatography graph. The low molecular weight region is the portion of the gel permeation chromatography graph where the logM value is 4 or less. When the integrated area of ​​the low molecular weight region is within this range, the processing load in the extruder does not increase when producing a separator using the polyethylene resin for secondary battery separators, improving processability and allowing a separator product with a uniform surface to be produced, while maintaining the rigidity of the final separator product.

[0024] The polyethylene resin may have an integrated area of ​​a high molecular weight region of 9.5% to 13.5%, for example, 9.8% to 13.2%, for example, 10.0% to 13.0%, of the total integrated area in a gel permeation chromatography graph. The high molecular weight region is the graph portion where the logM value is 6 or more, and when the integrated area of ​​the high molecular weight region is within this range, the polyethylene resin for secondary battery separators can be used to manufacture separators, improving processability in an extruder and allowing for the production of separator products with uniform surfaces. Furthermore, the rigidity of the final separator product is not reduced, resulting in a separator of excellent quality.

[0025] The polyethylene resin may be in powder form. The polyethylene resin may be formed as a single particle of a low molecular weight polymer having a viscosity average molecular weight of 180,000 g / mol to 700,000 g / mol and a high molecular weight polymer having a viscosity average molecular weight of 1,500,000 g / mol to 2,500,000 g / mol. Specifically, polymer particles polymerized in a first reactor are transferred to a second reactor and polymerized continuously so that the low molecular weight polymer formed in a first reactor and the high molecular weight polymer polymerized in a second reactor form a single particle. In this case, hydrogen may be injected as a molecular weight modifier into each reactor to achieve the target molecular weight.

[0026] The viscosity-average molecular weight of the low-molecular-weight polymer may be 180,000 g / mol to 700,000 g / mol, for example, 200,000 g / mol to 680,000 g / mol. When the viscosity-average molecular weight of the low-molecular-weight polymer is within this range, the processing load in the extruder does not increase when the polyethylene resin for secondary battery separator is used to manufacture the separator, improving processability and allowing the production of a separator product with a uniform surface, and preventing a decrease in the rigidity of the final separator.

[0027] The viscosity average molecular weight of the high molecular weight polymer may be 1,500,000 g / mol to 2,500,000 g / mol. When the viscosity average molecular weight of the high molecular weight polymer is within this range, the processability in an extruder during the manufacture of a separator using the polyethylene resin for a secondary battery separator is improved, allowing a separator product with a uniform surface to be produced, and the rigidity of the final separator product is not reduced.

[0028] The polyethylene resin may have a viscosity average molecular weight of 200,000 g / mol to 2,500,000 g / mol, for example, 400,000 g / mol to 1,000,000 g / mol, for example, 400,000 g / mol to 800,000 g / mol. When the viscosity average molecular weight of the polyethylene resin is within this range, the processability in an extruder is improved and gel formation in the final product, the separation membrane, is reduced, thereby ensuring a separation membrane product with a uniform surface and preventing a decrease in the mechanical strength of the separation membrane.

[0029] The polyethylene resin may have a melting temperature of 130°C to 138°C, for example, 133°C to 136°C. The melting temperature may be measured by a differential scanning calorimeter (DSC). When the melting temperature of the polyethylene resin is within this range, extrusion processability is improved and the mechanical strength of the final molded separator is not reduced.

[0030] The polyethylene resin may have an average particle size of 110 μm to 140 μm. Specifically, the polyethylene resin powder may have an average particle size of 110 μm to 140 μm, for example, 110 μm to 130 μm. When the average particle size is within this range, the flowability in the extruder hopper does not decrease, and a decrease in productivity due to a decrease in bulk density and poor extrusion do not occur.

[0031] The polyethylene resin may have a particle size distribution (SPAN) of 1.0 or less. Specifically, the polyethylene resin powder may have a particle size distribution of 1.0 or less, which may result in uniform melting characteristics in the extruder, reduced formation of unmelted gel, and improved flowability in the hopper.

[0032] The molecular weight distribution (MWD) of the polyethylene resin may be 4 to 6. The molecular weight distribution (MWD) is defined as Mw / Mn (where Mw corresponds to the weight average molecular weight and Mn corresponds to the number average molecular weight).

[0033] Another embodiment provides a method for producing a polyethylene resin for a separator for a secondary battery, the method including the steps of: injecting ethylene, a catalyst, and an organic solvent into a first reactor; injecting hydrogen into the first reactor; injecting a slurry produced by polymerization in the first reactor into a second reactor; additionally injecting ethylene and an organic solvent into the second reactor; and injecting hydrogen into the second reactor so that a ratio of an injection rate of hydrogen to an injection rate of ethylene into the second reactor is 0.01 to 0.30.

[0034] The method for producing a polyethylene resin for a secondary battery separator may include injecting ethylene, a catalyst, and an organic solvent into a first reactor and injecting a slurry produced by polymerization in the first reactor into a second reactor. Specifically, the first and second reactors are connected in series, and ethylene, a catalyst, and an organic solvent are injected into the first reactor. The slurry produced by polymerization in the first reactor is injected into the second reactor through a transfer pipe to ensure that the polymer produced in the first reactor and the polymer produced in the second reactor form a single particle. Then, in the second reactor, additional ethylene and an organic solvent are injected through another pipe, resulting in continuous polymerization in which the polymerization process is repeated.

[0035] In the first reactor, a polyethylene polymerization reaction can be carried out using a polyethylene polymerization catalyst and an organometallic cocatalyst. The polyethylene polymerization catalyst can be a solid complex titanium catalyst, for example, a magnesium-supported titanium catalyst.

[0036] The organometallic cocatalyst compound may be represented by the general formula MRn. M may be, for example, a Group II metal in the periodic table, including magnesium and calcium, a Group IIIA metal in the periodic table, including boron, aluminum, and gallium, or zinc. R represents an alkyl group having 1 to 20 carbon atoms, and n represents the valence of the metal component.

[0037] The organometallic compound may be, for example, an organometallic compound having one or more alkyl groups having 1 to 6 carbon atoms, such as trialkylaluminum including triethylaluminum and triisobutylaluminum, which may be used alone or in combination.

[0038] The organometallic compound may be an organoaluminum compound having one or more alkyl groups having 1 to 6 carbon atoms and further containing one or more halogen or hydride groups, such as ethylaluminum dichloride, diethylaluminum chloride, ethylaluminum sesquichloride, diisobutylaluminum hydride, or a combination thereof. For example, the organometallic compound may be triethylaluminum.

[0039] The method for preparing a polyethylene resin for a secondary battery separator may include injecting hydrogen as a molecular weight modifier into each of the first reactor and the second reactor to obtain a target molecular weight in each reactor.

[0040] When hydrogen is injected into the second reactor, the ratio of the hydrogen injection rate to the ethylene injection rate into the second reactor may be 0.01 to 0.30, for example, 0.01 to 0.25. When the ratio of the hydrogen injection rate to the ethylene injection rate into the second reactor is within the above range, a high molecular weight polymer having a viscosity average molecular weight of 1,500,000 g / mol to 2,500,000 g / mol may be produced in the second reactor.

[0041] When hydrogen is injected into the first reactor, the ratio of the hydrogen injection rate to the ethylene injection rate into the first reactor may be 0.02 to 0.35, for example, 0.02 to 0.30. When the ratio of the hydrogen injection rate to the ethylene injection rate into the first reactor is within the above range, a low molecular weight polymer having a viscosity average molecular weight of 180,000 g / mol to 700,000 g / mol may be formed in the first reactor.

[0042] The method for producing a polyethylene resin for a secondary battery separator further includes the steps of transferring the slurry produced by polymerization in the second reactor to a degassing process and drying the slurry transferred to the degassing process to obtain a powder-type polyethylene resin. That is, the slurry produced by polymerization in the second reactor is transferred to a degassing process through a pipe to discharge the final polyethylene resin produced in the second reactor. After the degassing process, the slurry is separated into an organic solvent and polyethylene resin in a separation process, and finally passes through a drying process to produce an ultra-high molecular weight polyethylene resin, for example, a powder-type ultra-high molecular weight polyethylene resin.

[0043] In the method for preparing the polyethylene resin for secondary battery separators, the polymerization reactions in the first and second reactors can be performed by a liquid slurry polymerization method in the presence of an organic solvent, which can be carried out in the absence of oxygen, water, and other compounds that may act as catalyst poisons.

[0044] The organic solvent may be an alkane, such as pentane, hexane, heptane, n-octane, isooctane, or a combination thereof; a cycloalkane, such as cyclohexane, methylcyclohexane, or a combination thereof; an alkyl aromatic, such as toluene, xylene, ethylbenzene, isopropylbenzene, ethyltoluene, n-propylbenzene, diethylbenzene, or a combination thereof; a halogenated aromatic, such as chlorobenzene, chloronaphthalene, orthodichlorobenzene, or a combination thereof; or a combination thereof. For example, the organic solvent may be a hydrocarbon solvent having 4 to 6 carbon atoms, such as hexane.

[0045] Yet another embodiment provides a separator for a secondary battery, comprising the polyethylene resin. In yet another embodiment, a secondary battery is provided, comprising: a positive electrode; a negative electrode; and a separator for the secondary battery disposed between the positive electrode and the negative electrode. The structure, materials, and manufacturing method of the secondary battery are well known in the art, and therefore, description thereof will be omitted.

[0046] Specific examples of the present invention will be presented below. However, the examples described below are merely for the purpose of specifically illustrating or explaining the present invention and should not be construed as limiting the present invention. Furthermore, since the details not described herein can be sufficiently inferred by those skilled in the art, a description thereof will be omitted.

[0047] (Production of polyethylene resin) Example 1 Phase 1 A 1 L reactor equipped with a mechanical stirrer was purged with nitrogen, and then 25 g of magnesium chloride (MgCl), 300 ml of toluene, and 100 ml of n-butanol were added. While stirring, the temperature was raised to 65°C for 1 hour and maintained at this temperature for 2 hours to obtain a uniform magnesium halide compound solution.

[0048] Phase 2 The temperature of the magnesium halide compound solution prepared was cooled to 40°C, and 70 ml of titanium tetrachloride (TiCl4) was slowly injected over a period of 1 hour. After the injection was completed, the temperature of the reactor was raised to 60°C for 1 hour while stirring at 350 rpm, and the mixture was aged for an additional 1 hour. When the entire process was completed, the reactor was stopped, the solid components were completely submerged, and the supernatant liquid was removed. The solid component (catalyst precursor) in the reactor was then washed with 200 ml of hexane.

[0049] 200 ml of hexane, 60 ml of titanium tetrachloride (TiCl4), and 8 ml of ethyl benzoate were added to the prepared catalyst precursor, and the reactor temperature was raised to 70°C over 1 hour while stirring at 350 rpm, followed by aging for 2 hours. Upon completion of the entire process, the reactor was stopped, the solid component was completely submerged, and the supernatant was removed. The prepared solid component, polyethylene polymerization catalyst (A), was washed six times with 200 ml of hexane.

[0050] Phase 3 Two 150-liter continuous stirred tank reactors (CSTRs) (reactor 1 and reactor 2) equipped with internal temperature and pressure regulators and agitators were connected in series for continuous polymerization. Ethylene (2.5 kg / hr), hexane (22.5 kg / hr), and the polyethylene polymerization catalyst (A) prepared above were continuously injected into the first reactor at 0.1–0.2 g / hr depending on activity, and the mixture was stirred at 180 rpm. The hexane injection rate was adjusted to ensure a reaction residence time of 2 hours. Hydrogen for molecular weight control was injected at an injection rate adjusted by measuring the high load melt index (HLMI). Triethylaluminum dissolved in 11 wt. % hexane was used as a cocatalyst. The discharge and liquid level were controlled to maintain a volume of 70 L in the first reactor. The continuously discharged hexane slurry was transferred to the second reactor connected in series, and the liquid level was controlled to maintain the same reaction volume and residence time in the second reactor. Ethylene was injected at 1.5 kg / hr to continue polymerization. The reaction temperatures were maintained at 80°C in the first reactor and 78°C in the second reactor, and the reaction pressure was 3.8 kgf / cm. 2 and 3.0 kgf / cm 2 Hydrogen was injected into the first reactor so that the ratio of the hydrogen injection rate to the ethylene injection rate into the first reactor was 0.28, and hydrogen was injected into the second reactor so that the ratio of the hydrogen injection rate to the ethylene injection rate into the second reactor was 0.014. The mixing ratio of the slurries in each reactor was maintained at 53 for the first reactor and 47 for the second reactor. The slurry solution continuously discharged by the liquid level control was subjected to a degassing process and a separation process to produce ultra-high molecular weight polyethylene powder in the form of a wet cake. Subsequently, a continuous drying process was performed to produce ultra-high molecular weight polyethylene resin in the form of a particulate powder.

[0051] Example 2 A polyethylene resin was produced in the same manner as in Example 1, except that in step 3 of Example 1, the polymerization reaction was carried out with the hydrogen injection rate ratios in the first reactor and the second reactor set to 0.025 and 0.22, respectively, and the slurry mixing ratios in the first reactor and the second reactor set to 47 and 53, respectively.

[0052] Comparative Example 1 In the third stage of Example 1, one 150-liter CSTR reactor equipped with an internal temperature controller, a pressure controller, and an agitator was used, and the polymerization temperature was 80°C and the polymerization pressure was 4.7 kgf / cm. 2 A polyethylene resin was prepared in the same manner as in Example 1, except that the polymerization reaction was carried out with the ratio of the injection rate of the ethanol and hydrogen being 0.103.

[0053] Comparative Example 2 A polyethylene resin was prepared in the same manner as in Example 1, except that in the third step of Example 1, the polymerization reaction was carried out with the hydrogen injection rate ratios of the first reactor and the second reactor set to 0.36 and 0.006, respectively.

[0054] Comparative Example 3 A polyethylene resin was produced in the same manner as in Example 1, except that in step 3 of Example 1, the polymerization reaction was carried out with the hydrogen injection rate ratios in the first reactor and the second reactor set to 0.22 and 0.004, respectively, and the slurry mixing ratios in the first reactor and the second reactor set to 59 and 41, respectively.

[0055] Evaluation: Measurement of the physical properties of polyethylene resin The polyethylene resins prepared in Examples 1 and 2 and Comparative Examples 1 to 3 were measured for the following physical properties, and the results are shown in Table 1 below.

[0056] Reaction pressure (kgf / cm 2 ) The pressure was measured using a pressure gauge installed in the reactor. high load melt index (HLMI) (g / 10 min) Measured according to ASTM D1238 at 190°C with a load of 21.6 kg.

[0057] Viscosity average molecular weight (g / mol) The viscosity average molecular weight was calculated from the intrinsic viscosity [η] according to ASTM D4020. In the case of polymers, viscosity can provide useful information in dilute solutions; the viscosity of a polymer divided by the viscosity and concentration of the solution is called the specific viscosity, and the extrapolated value of the specific viscosity when the polymer concentration becomes zero is defined as the intrinsic viscosity (IV). For linear polymers, the intrinsic viscosity value is primarily affected by the size of the polymer, so it has a high correlation with molecular weight; in the case of ultra-high molecular weight polyethylene, the Margolies equation is widely used.

[0058] Viscosity average molecular weight=5.37×10 4 ×[η] 1.49 [η]=intrinsic viscosity(dl / g) Melting temperature (Tm) (℃) The melting temperature (Tm) was measured by differential scanning calorimetry (DSC). After heating to 200°C and maintaining an isothermal state for 10 minutes, the temperature was lowered to 30°C at a rate of 10°C per minute to remove the thermal history, and then the temperature was increased to 200°C at a rate of 10°C per minute to measure the melting temperature (Tm).

[0059] Average particle size and distribution (SPAN) The average particle size of the polymer was measured using a polymer particle analyzer (MALVERN MASTER SIZE X PARTICLE ANALYSER) according to ISO 13320-2. The average particle size is D(v, 0.5), and the particle size distribution (span) is expressed as (D(v, 0.9) - D(v, 0.1)) / D(v, 0.5). Here, D(v, 0.5) represents the particle size represented by 50% of the sample, while D(v, 0.9) and D(v, 0.1) represent the particle sizes represented by 90% and 10% of the sample, respectively. The smaller the distribution number, the narrower the distribution.

[0060] Bulk density (g / cc) Measured according to ASTM D1895-96. Melting time (s) 3 g of powder, 7 g of oil, and a magnetic bar were placed in a 50 ml vial, and the melting state was monitored over time using a magnetic stirrer set at 180°C and 1800 rpm to measure the time until complete melting.

[0061] Molecular weight distribution (MWD) and integrated area (%) of low / high molecular weight regions The molecular weight distribution (MWD) was defined as Mw / Mn (where Mw is the weight-average molecular weight and Mn is the number-average molecular weight) from the data obtained through gel permeation chromatography (GPC) analysis. Based on the GPC analysis data, the ratio of the integrated area of ​​the low molecular weight region to the integrated area of ​​the high molecular weight region was calculated to determine the low / high molecular weight region range (see Figure 1). The equipment used for GPC analysis was an Agilent PS-GPC220. The solvent used was 1,2,4-TCB (1,2,4-trichlorobenzene), and the measurement temperature was 160°C.

[0062] Extrusion temperature (°C) and pressure (bar) An SM Platek TEK-30 twin screw extruder was used and temperature and pressure gauge readings on the extruder were recorded.

[0063] Sheet appearance grade The surface of the sheet extruded through a T-die using a TEK-30 twin screw extruder manufactured by SM Platek was visually inspected and graded.

[0064] Grade: 0-10, 0: very good, 10: very bad Grades 3 and above are considered defective products and cannot be used. Pin puncture strength (g) Using a KES-G5 instrument manufactured by Kato Tech, Japan, the puncture strength was measured at a speed of 10 mm / sec using a tip with a terminal diameter of 1 mm according to ASTM D-4833.

[0065] [Table 1]

[0066] From Table 1, it can be seen that Examples 1 and 2 exhibited lower appearance ratings than Comparative Examples 1 to 3, while the separator sheets made with the polyethylene resins of Examples 1 and 2 exhibited much improved appearances compared to the separator sheets made with the polyethylene resins of Comparative Examples 1 to 3. That is, the separator sheets made with the polyethylene resins of Comparative Examples 1 to 3 exhibited sheet appearance ratings of 3 or higher, making them unusable due to product defects, whereas the separator sheets made with the polyethylene resins of Examples 1 and 2 exhibited sheet appearance ratings of less than 3, indicating that they could be used as separators without any problems.

[0067] 1, this is because the polyethylene resins of Examples 1 and 2 satisfy the range of physical properties for a polyethylene resin for a secondary battery separator according to an embodiment, including the ratio of the integrated area of ​​the low molecular weight region and the integrated area of ​​the high molecular weight region to the total integrated area in a gel permeation chromatography graph, thereby increasing the melting rate of the polyethylene resin in the extruder. That is, the time it takes for the powder-type polyethylene resin to melt into oil during extrusion is reduced, resulting in rapid and uniform melting and reduced gel formation within the sheet.

[0068] Therefore, when using the polyethylene resin for a separator of a secondary battery according to an embodiment, by optimizing the content of low molecular weight polyethylene and high molecular weight polyethylene, the processability in the extruder can be improved, and a separator product with a uniform surface can be produced.

[0069] Furthermore, with regard to the strength of the separation membrane, Examples 1 and 2 exhibited slightly higher puncture strength values ​​than Comparative Examples 1 to 3. That is, it can be seen that the rigidity and mechanical strength of the separation membranes made from the polyethylene resins of Examples 1 and 2 were not reduced compared to the separation membranes made from the polyethylene resins of Comparative Examples 1 to 3, and were slightly higher.

[0070] In addition, in the case of polyethylene resins, a broader molecular weight distribution generally results in excellent processability but poor rigidity. However, in the case of the polyethylene resins of Examples 1 and 2, the molecular weight distribution was broadened by having an optimal ratio of the integrated areas of the low molecular weight region and the high molecular weight region, and it was found that maintaining an optimal low / high molecular weight content prevented a decrease in the rigidity of the final separator membrane.

[0071] Therefore, when using a polyethylene resin for a separator of a secondary battery according to an embodiment, by optimizing the content of low molecular weight polyethylene and high molecular weight polyethylene, the processability in an extruder can be improved, a separator product with a uniform surface can be produced, and the rigidity of the final separator can be maintained.

[0072] Although the preferred embodiment of the present invention has been described above, the present invention is not limited thereto, and various modifications can be made within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and it goes without saying that these also fall within the scope of the present invention.

Claims

1. A method for producing a polyethylene resin for a secondary battery separator, comprising: injecting ethylene, hexane, and a polyethylene polymerization catalyst into a first reactor; injecting hydrogen and a cocatalyst into the first reactor; injecting the slurry produced by polymerization in the first reactor into a second reactor; additionally injecting ethylene, hexane, and a polyethylene polymerization catalyst into the second reactor; maintaining the reaction temperature and reaction pressure in the first reactor and the second reactor at predetermined values; injecting hydrogen into the first reactor so that the ratio of the injection rate (kg / hr) of hydrogen to the injection rate (kg / hr) of ethylene injected into the first reactor is 0.02 to 0.35; injecting hydrogen into the second reactor so that the ratio of the injection rate (kg / hr) of hydrogen to the injection rate (kg / hr) of ethylene into the second reactor is 0.01 to 0.30; The polyethylene resin for secondary battery separator has a weight average molecular weight (Mw) of 350,000 g / mol to 500,000 g / mol as determined by gel permeation chromatography; the integrated area of ​​the low molecular weight region is 1.3% to 2.4% and the integrated area of ​​the high molecular weight region is 9.5% to 13.5% of the total integrated area of ​​the gel permeation chromatography graph; The low molecular weight region is a graph portion in which the log M (where M corresponds to the molecular weight of the polymer passing through the column in the gel permeation chromatography) value is 4 or less in the gel permeation chromatography graph, and the high molecular weight region is a graph portion in which the log M value is 6 or more. A manufacturing method for polyethylene resin for secondary battery separators.

2. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator is in a powder form.

3. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator has a viscosity average molecular weight of 200,000 g / mol to 2,500,000 g / mol.

4. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator has a melting temperature of 130 ° C to 138 ° C.

5. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator has an average particle diameter of 110 μm to 140 μm.

6. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator has a particle size distribution (SPAN) of 1.0 or less.

7. 2. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the molecular weight distribution (MWD) value of the polyethylene resin for a secondary battery separator is 4 to 6.

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